Dolby Pro Logic was designed to decode soundtracks encoded with Dolby Surround. The surround sound processing technology was developed by Dolby Laboratories. In 1976, Dolby developed Dolby Surround Stereo for analog cinema sound systems, which was adapted for home use in 1982 as Dolby Surround. This happened when HiFi capable consumer VCRs were introduced that later got replaced by the improved Pro Logic system in 1987.
“Dolby Surround” refers to the encoding technology or matrix-encoded soundtrack and “Pro Logic” refers to the decoding technology or processor. Dolby Pro Logic is the domestic equivalent (consumer version) of the theatrical Dolby Stereo technology that was used in movie cinemas in 1970s and 1980s.
The technology is based on matrix technology. It involves the matrix-encoding of four channels of sound into an ordinary stereo (two channel) sound track, when a Dolby Surround soundtrack is created. The center channel is encoded by placing it equally in the left and right channels while the rear channel is encoded using phase shift techniques. Systems with a Pro Logic decoder/processor unfold the sound into the original 4.0 surround while systems lacking the decoder play back the audio as standard Stereo.
Dolby Digital decoders incorporate Dolby Surround Pro Logic decoder for digital stereo signals that carry matrix-encoded Dolby Surround. Dolby Pro Logic II was introduced in 2000 that processed any high quality stereo signal source into five full frequency channels. The next version, Dolby Pro Logic IIx takes stereo, Dolby Surround and Dolby Digital 5.1 source material and up-converts it to 6.1 or 7.1 channel surround sound. Dolby Pro Logic IIz has an addition of a height component that expands a 5.1 or 7.1 system to 7.1 Height or 9.1.